The invention has an important application in connecting single mode optical fibres to optical components comprising waveguides formed on substrates.
Successful introduction into optical communications systems of optical components on planar substrates, such as, for example, optical waveguides on lithium niobate substrates, often depends among other criteria on the ability to provide interconnection of such components with optical fibres. The interconnection should not only have low losses and be capable of withstanding reasonable mechanical shocks, but should also be advantageously simple and reliable to make.
Where the optical component concerned is, or is similar in structure to, an optical waveguide on a substrate, such an interconnection is particularly difficult to provide. The waveguide is embedded in a surface of the substrate, the end-faces of the substrate being co-planar with the terminations of the waveguide. Because there is no close structural similarity between an optical fibre end and the end-portion of a substrate, the two components being of substantially differing dimensions and cross-sections, special techniques have been developed for making the interconnections.
An arrangement which has produced acceptably low losses between an optical fibre and a waveguide on a substrate is that called "end-fire" coupling. In this arrangement the fibre core is physically aligned with the waveguide and optically coupled directly thereto with no intervening components. Such an arrangement is disclosed by:
ALFERNESS, R. C., RAMASWAMY, V. R., KOROTKY, S. K., DIVINO, M. D., and BUHL, L. L.: "Efficient single-mode fibre to titanium diffused lithium niobate waveguide coupling for I=1.32 .mu.m". IEEE J. Quantum Electron., 1982, QE-18, pp 1807-1813. The technique achieves low losses not only through precise alignment of the fibre end with the waveguide end but also by closely matching the mode-pattern of the wave guide to that of the fibre.
The above technique does not however address the problem of how to fix the fibre end relative to the waveguide while preserving the low losses, and how to do so in a manner suitable for component packaging.
The absence of a readily usable manner of providing the connections between the fibre and waveguide presents, in fact, a major obstacle to the use of the end-fire coupling technique in the packaging of waveguide components, because misalignment resulting in a significant increase in the coupling loss, can easily occur. It should be borne in mind that the fibre cores of single-mode fibres typically have diameters of between 0.005 and 0.01 mm. The waveguides are of similar dimensions, and consequently misalignment of only a few micrometres is sufficient to cause a serious increase in the connection loss.
The search for techniques of providing satisfactory connection has attracted a considerable amount of effort as witnessed by the following disclosures, all of which refer to techniques for connecting the fibre end to a waveguide in the correct spatial relationship:
SHEEM S. K., GIALLEORANI T. G.: "Two-dimensional silicon grooves for altitudinal alignment in fibre end-butt coupling". Optics Letts., 1978, 3, pp 73-752.
BULMER C. H., SHEEM S. K., MOELLER R. P., and BURNS W. K.: "Fabrication of flip-chip optical couplers between single-mode fibre and lithium niobate channel waveguides". IEEE Trans. on Comps. Hybrids and Manuf. Tech., 1981, CHMT-4, pp 350-355.
HSU H. P., MILTON A. F., and BURNS W. K.: "Multiple fibre end fire coupling with single-mode channel waveguides". Appl. Phys. Lett., 1978, 33(7), pp 603-605.
RAMER O. G., NELSON C., and MOHR C.: "Experimental integrated circuit losses and pigtailing of chips" IEEE J. Quantum Electron., 1981, QE-17, pp 970-974.
ARDITTY H. J., BETTINI J. P., GRAINDORGE P., LEFEVRE H. C., PAPUCHON M., BOURBIN Y., VATOUX S., BERARD D., and COLOMBIN J. Y.: "Test results on an integrated fibre-optics gyroscope brass board". IEE conf. Optical Fibre Sensors, Apr. 26-28, 1983, London, UK.
All of the aforementioned references use silicon V-grooves to provide lateral and angular fibre to waveguide alignment, and rely on the use of fibres with highly concentric cores and constant diameters to achieve satisfactory optical coupling. To produce coupling which is independent of concentricity and diameter however, additional vertical alignment is required. One method of achieving this is to use a tapered adjustment fibre lying in a transverse groove under the fibre to be coupled, which provides submicron vertical alignment, as described for example by SHEEM S. K. et al and by BULMER C. H. et al of the above references. Once aligned, the fibre is then fixed into its groove with an epoxy resin.
Although this form of alignment and fixing provides an interconnection of relatively good mechanical strength, it does not surmount the problem mentioned above, i.e. the introduction of misalignment at the fixing stage.
The present invention aims to provide a simple and practical method of, and arrangement for, affixing single mode fibres to waveguides on planar substrates which, moreover, do not need to rely on core concentricity or known diameter of the optical fibres.
It should be noted that the combination of a waveguide and an associated substrate will be referred to hereinafter as a waveguide substrate.
According to one aspect of the present invention, a method of affixing a single-mode optical fibre to a waveguide substrate in optical alignment with a waveguide thereon comprises the steps of:
(a) preparing an end of the fibre such as to have a planar end-face substantially normal with respect to the fibre axis,
(b) extending the potential area of contact between the fibre end-face and the waveguide substrate by mounting on the surface of the substrate in which the waveguide is embedded a support block which straddles the waveguide and which has an end-face substantially co-planar with the termination of the waveguide,
(c) optically aligning the fibre end and the waveguide, and
(d) affixing the end-face of the optical fibre with transparent adhesive to the waveguide substrate and block such that the optical fibre is butt-jointed to the substrate and block by the transparent adhesive to maintain axial alignment of the optical fibre and the waveguide.
According to another aspect of the present invention, a connection between a single mode optical fibre and a waveguide on a waveguide substrate comprises:
(a) a waveguide substrate having one or more waveguides embedded therein near one surface thereof,
(b) a single mode optical fibre, having a planar end-face substantially normal with respect to the fibre axis, in optical alignment with the waveguide,
(c) a support block having an end-face substantially co-planar with the termination of the waveguide and being mounted on the surface of the substrate in which the waveguide is embedded so as to straddle the waveguide,
(d) and transparent adhesive affixing the end-face of the optical fibre to both the waveguide substrate and block said optical fibre, waveguide substrate and block being disposed such that the optical fibre is butt-jointed to the substrate and block by the transparent adhesive to maintain axial alignment of the optical fibre and the waveguide.
It should be noted that the term "transparent" is to be understood as meaning "transparent at the wavelength of operation". Thus, for use at an infrared wavelength of 1500 nm, for example, it is important that the adhesive be transparent at that wavelength, but not necessarily be so as visible wavelengths. Equally, the term "refractive index" below is to be understood as meaning "refractive index at the wavelength of operation".
The adhesive is conveniently a light curable adhesive, and preferably a white light curable adhesive.
Connections between single mode fibres and waveguide substrates made in accordance with the present invention have been shown to give an acceptably strong mechanical joint between the fibre and the waveguide substrate, without introducing unacceptably high coupling losses between the fibre core and the waveguide.
The interface or contact area of the support block with the waveguide should have a refractive index which does not greatly exceed the refractive index of the waveguide, since a region of too large a refractive index next to the waveguide would cause increased losses in the wave guide. Therefore, the interface or contact area preferably has a refractive index equal to or lower than the refractive index of the waveguide. This requirement can be readily complied with by affixing the support block to the substrate by means of an adhesive of suitable refractive index, so that the interface is formed by the adhesive interposed between the waveguide and the support block.
The support block conveniently comprises similar, and preferably the same, material as the waveguide substrate.
In the case where there are two or more waveguides on a substrate, the support block may extend over several adjacent waveguides. In this case also two or more fibre to waveguide connections may be made to the same substrate by the method of the present invention.
The adhesive used for affixing the support block may conveniently be the same as that used for fixing the fibre to the waveguide.
Polishing the edges of the substrate and the support block separately, and mounting the support block by the method of the present invention after deposition of the desired device structure (e.g. metallisation) is substantially complete, has appreciable advantages over mounting the support block early during fabrication and polishing the end face of the support block and the substrate together. Firstly, a polishing step late in the fabrication process is undesirable because of the possibility of causing damage to the finished device. Damage caused at a late stage of device fabrication can considerably reduce the yield and hence increase the cost of devices because of the waste in processing time already spent on the devices. Moreover, it has been found that the greater accuracy in alignment provided by polishing after assembly is unnecessary, as any minor (to within about plus or minus 5 .mu.m) misalignment between the edges of the block and the substrate can be adequately compensated for by variations in the thickness of the adhesive. The only essential alignment is laterally, and can be achieved by the method of present invention.